This invention relates to an inverter for converting direct current to alternating current and, in particular, to a high voltage inverter that uses resonance effects to achieve a large voltage gain.
In portable electronic devices, instrument panels in vehicles, and other applications where the power source is a low voltage battery, a cold cathode fluorescent lamp is driven by an inverter that converts low voltage direct current into high voltage alternating current. In order for a cold cathode fluorescent lamp to glow sufficiently, a peak-to-peak voltage in excess of about one thousand volts is necessary. As known to those of skill in the art, the actual voltage depends on the construction of the lamp and, in particular, the length and diameter of the lamp.
The available DC voltage limits the voltage that can be produced by an inverter. Converting from six volts or less to one thousand volts or more is difficult without a transformer. A problem with a transformer is that a transformer must be custom wound for each circuit. This increases the cost of the circuit and makes it difficult to modify the circuit without requiring a new transformer design.
To increase the output voltage without a transformer, the prior art teaches using what is known as a xe2x80x9cflybackxe2x80x9d inverter in which the energy stored in an inductor is supplied to a storage capacitor as a small current at high voltage. Ignoring the resistance of the wire, the voltage across an inductor is proportional to Lxc2x7xcex4i/xcex4t. By abruptly turning off the current through an inductor, a low voltage at high current is converted into a high voltage at low current.
It is also known in the art that high voltages can be generated in resonant circuits. For example, U.S. Pat. No. 5,587,629 (Gornstein), discloses a power supply for a photoflash. The power supply includes one half of a twin-T filter in the feedback path of an amplifier. The output is taken across the stem of the T.
A problem with the Gornstein circuit is the unrealizable Q of the components, particularly the inductor. A Q of 200 to 400 is attainable in an inductor only by careful construction on expensive cores. What is desired is a resonant inverter that can use components having standard values rather than using custom designed components. Standard components greatly reduce the cost of the inverter but have the disadvantage of requiring that the circuit be tolerant of variations in component values.
The Gornstein circuit stores pulses of rectified AC on a capacitor for discharge through a flash lamp. What is desired is a circuit capable of providing continuous current through a cold cathode fluorescent lamp. Such circuits typically include a transformer, which adds significantly to the cost and bulk of the circuit. Each application, i.e. each lamp, typically requires a different transformer, which makes the circuit expensive to design and manufacture. A transformer is necessary because the starting voltage of a cold cathode fluorescent lamp is typically at least twice as high as the operating voltage of the lamp. Commercially available inverters for cold cathode fluorescent lamps produce 70-1500 volts rms.
In view of the foregoing, it is therefore an object of the invention to provide an inverter for converting low voltage direct current into high voltage alternating current without high voltage switching transistors.
Another object of the invention to eliminate the transformer from an inverter for cold cathode fluorescent lamps.
A further object of the invention is to provide an inverter using resonance effects to convert a voltage of 12 volts DC or less into a voltage of 500 volts rms AC or more.
Another object of the invention is to provide an inverter using a high voltage oscillator constructed from standard components.
The foregoing objects are achieved by this invention in which the inverter includes a fourth order impedance network coupled between the output and the input of an amplifier, causing the amplifier to oscillate and produce a high voltage at an output of the impedance network. The impedance of the output portion of the impedance network is preferably at least ten times the impedance of the input portion of the impedance network at the frequency of oscillation.